The invention relates to an apparatus for determining the postures of a flying craft.
To fly an aerodyne, it is necessary to know its postures.
The postures are by definition the three Euler angles "psgr", xcex8, xcfx86, or, respectively, heading, attitude and inclination, which define the orientation of the reference frame tied to the aerodyne with respect to the local geographical reference frame. The local geographical reference frame (or trihedron) is an orthonormal trihedron one axis of which is along the local vertical (downwards) and whose other two axes are along the local north and east directions. The reference frame tied to the aerodyne is an orthonormal trihedron whose X axis, or axis of roll, is colinear with the fuselage of the aerodyne, whose Y axis, or axis of pitch, is perpendicular to its plane of symmetry, and whose Z axis, or axis of yaw, is included in the plane of symmetry.
The postures are determined with the aid of a so-called strap-down gyrometric device consisting of at least three mutually integral gyrometers integral with the structure of the aircraft. Each of them provides a component, along its axis, of the vector {right arrow over (xcexa9)} of instantaneous rotation of the aerodyne with respect to an inertial frame of reference. This device makes it possible to measure the three components of {right arrow over (xcexa9)} in the reference frame X, Y, Z tied to the aerodyne. These components are, respectively, the roll p about the X axis, the pitch q about the Y axis and the yaw r about the Z axis.
These three components are integrated, for example by the so-called method of quaternions. Integration provides quaternions of postures, from which is deduced, with the initial orientation of the aerodyne, a matrix of postures followed by the Euler angles.
The measurement signals delivered by the gyrometers exhibit defects referred to as drifts. Since these signals are integrated, the errors in the postures increase with time. This is why, conventionally, to determine the postures, use is made, in addition to the gyrometers, of accelerometers which are used to correct the results provided by the integration of the signals from the gyrometers. This correction is performed in a slaving loop.
The accelerometers integral with the strap-down gyrometric device can provide, by virtue of gravity, the attitude postures and inclinational of the aerodyne; however the signals which they deliver are marred with errors which make the results difficult to utilize as they stand. Conversely, the combining, in a slaving loop, of the signals provided by gyrometers and accelerometers makes it possible to compensate for the drifts of the gyrometers whilst preserving their advantage which is to provide results with relatively low short-term noise.
In known apparatuses, comparison between the gyrometric and accelerometric signals constitutes a complex operation since the signals from the accelerometers are integrated in such a way as to represent velocities and the latter are compared with references; this comparison serves to formulate the correction of the gyrometric device. Integration calls upon postures emanating from the gyrometric device.
The invention provides an apparatus in which the corrections are performed in a simple and reliable manner.
The apparatus according to the invention is characterized in that the comparison is performed between, on the one hand, a vertically directed vector provided by the gyrometric device and, on the other hand, a vertically directed vector provided by the accelerometers. Preferably, the comparison consists in performing the vector product of these two vectors.
The vertically directed vector provided by the accelerometers is colinear with the acceleration due to gravity, during the phases of unaccelerated rectilinear flight. Thus, it is not necessary to perform an integration of the accelerometric signals. Moreover, in the gyrometric device, the vertically directed vector constitutes the last column of the posture matrix obtained with this device. Furthermore, the two vertically directed vectors can be expressed in the same reference frame tied to the aerodyne.
It is recalled here that the posture matrix is a 3xc3x973 matrix in which the first column represents the direction of north in the XYZ reference frame of the aerodyne, and the second and third columns respectively represent the east and vertical directions, also in the XYZ reference frame.
It should furthermore be noted that the vector product is isotropic, that is to say independent of the reference frame.
When employing a vector product, the corrections to be made are also of great simplicity since the modulus of the vector product is proportional to the sine of the angle of the rotation which brings the vertically directed vector provided by the gyrometric device into coincidence with the vertically directed vector provided by the accelerometric device. Moreover, the direction of the vector product gives the axis of this rotation.
If the comparison consists in performing a vector product, further arrangements may be necessary. Specifically, a vector product being, by definition, perpendicular to each of the vectors of the product, its component is zero along the vertical direction, and if one wishes to estimate the vertical component of the drift of the gyrometric device, it is necessary to have further data available.
In one embodiment of the invention, to obtain an estimate of the vertical component of the drift of the gyrometric device, an orthogonal projection of the vector {right arrow over (xcexa9)} of components p, q, r is performed, which vector is provided by the gyrometers, after compensating for the drift (by the abovementioned slaving), onto the vertical direction provided by the gyrometric device.
To obtain this projection, it is sufficient to perform the scalar product of the vector {right arrow over (xcexa9)} times the unit vector {right arrow over (U)}g of the vertical direction provided by the gyrometric device. This projected vector can be used, in an integral-return slaving, to compensate for the vertical component of the drift of the vector {right arrow over (xcexa9)} provided by the gyrometric device.
The scalar product {right arrow over (xcexa9)}.{right arrow over (U)}g represents the drift of the gyrometer in the vertical direction only if the aircraft is in the rectilinear flight phase, that is to say if it is not turning about the vertical direction. Thus, in one embodiment, the turning phases are detected and the yaw slaving loop (making it possible to compensate for the vertical component of the drifts of the device) is opened during the turning phases.
Likewise, the accelerometric device provides a vector representing the acceleration due to gravity only during the phases of unaccelerated rectilinear flight. It is therefore preferable to open at least some slaving loops of the gyrometric device during the turning and/or acceleration phases. The roll slaving loop is opened in the turning phases and the pitch slaving loop is opened during the acceleration phases. During accelerated turns, it is preferable to open the roll loop, the pitch loop remaining closed.
In one variant, to obtain an estimate of the vertical component of the drift of the gyrometric device, instead of using yaw-wise slaving, use is made of slaving in terms of heading, that is to say that the direction of north expressed in the aerodyne reference frame, provided by the gyrometric device, is slaved to the magnetic north provided by an external sensor such as a magnetometer.
Preferably, the discrepancy in heading is represented by a vector whose direction corresponds to the vertical of the gyrometric device so that it does not come into conflict with the abovementioned vector product which, by definition, it perpendicular to the vertical direction provided by the gyrometric device. Accordingly, it is possible to use the vector product of a vector {right arrow over (U)}m representing the direction of magnetic north (provided by a magnetometer) times: a vector {right arrow over (b)}1 representing the direction of north provided by the gyrometric device in the first column of the posture matrix. This vector product is then projected onto the vertical direction {right arrow over (b)}3 provided by the gyrometric device.
The invention thus provides an apparatus for determining postures, in particular the attitude and the inclination of an aerodyne, comprising:
a strap-down gyrometer device providing the components of the vector of instantaneous rotation of the aerodyne in a reference frame tied to the aerodyne,
means for calculating, from gyrometric measurements, a matrix of postures defining the orientation of the aerodyne with respect to the local geographical reference frame,
accelerometers integral with the gyrometric device, and
means for comparing data calculated from the accelerometers with data calculated from the gyrometers so as to provide corrections for compensating for the errors or drifts of the gyrometric device;
this apparatus is characterized in that the comparison is performed between a vertically directed vector {right arrow over (U)}g provided by the gyrometric device and a vertically directed vector {right arrow over (U)}a provided by the accelerometers.
The apparatus comprises, in one embodiment, means for calculating, from the accelerometric measurements, the direction of the vertical in a reference frame tied to the aerodyne. The vertically directed vectors are, for example, defined by their coordinates in a reference frame tied to the aerodyne.
The vertically directed vector provided by the gyrometric device may be obtained from the last column of the posture matrix.
Preferably, the vertically directed vectors have unit length.
In one embodiment, the means of comparison perform the vector product of the two vertically directed vectors.
The apparatus comprises a slaving loop of proportional type which, preferably, exhibits a gain limited to a predetermined maximum value.
In one embodiment, the apparatus comprises a slaving loop with integrator for compensating for drifts.
When the vector product of the two vertically directed vectors is performed, it is advantage for the apparatus to comprise a means for performing a correction along the vertical direction. In this case, the latter means comprises, for example, a means for calculating the projection, onto the vertical direction, of the instantaneous rotation vector provided by the gyrometric device, the vertical direction onto which the projection is performed being provided by the gyrometric device, and the instantaneous rotation vector which is projected being corrected by the slaving loop.
According to one embodiment, the vertically directed vector is provided, on the one hand, by the accelerometers and, on the other hand, from the velocity of the aerodyne as delivered, for example, by anemometric means.
In the latter case, the vertical direction is determined from the following equation:
{right arrow over (F)}d/m=d{right arrow over (V)}/dt=+{right arrow over (xcexa9)}{circumflex over ( )}{right arrow over (V)}xe2x88x92{right arrow over (F)}c/m,
{right arrow over (F)}d being the force of gravity, directed vertically, {right arrow over (V)} the velocity vector of the aerodyne determined by the anemometric means, {right arrow over (xcexa9)} the vector of instantaneous rotation of the aerodyne determined by the gyrometric device, and {right arrow over (F)}c the resultant of the contact forces as determined by the accelerometers, and m the mass of the aerodyne.
In one embodiment, the apparatus comprises a means for slaving the gyrometric heading to the magnetic heading.
In the latter case, and when the vector product of the vertically directed vectors is performed, the slaving of the heading is determined from the correction vector satisfying the following equation:             ϵ      →        c    =            [                        (                                                    U                →                            m                        ⩓                                          b                →                            1                                )                ⁢                              b            →                    3                    ]        =                  (                                            U              →                        m            :                    ·                                    b              →                        2                          )            ·                        b          →                3            
in which equation {right arrow over (U)}m is a vector representing the direction of the terrestrial magnetic field, preferably of unit amplitude, and the vectors {right arrow over (b)}1,{right arrow over (b)}2,{right arrow over (b)}3 are three vectors whose coordinates are provided by the columns of the posture matrix delivered by the gyrometric device.
According to one embodiment, the apparatus comprises closed loops for slaving the postures using the correction signals provided by the comparison means, and means for opening at least one loop outside of the rectilinear-flight and constant-velocity phases of the aerodyne.
In the latter case, the opening of the slaving loops may occur when the discrepancy between the vertically directed vector provided by the gyrometric device and the vertically directed vector provided by the accelerometers exceeds a predetermined value. In the event of a turn, for example, a roll slaving loop remains open, while a corresponding pitch slaving loop remains closed.
The rate of turn may be used to distinguish between rectilinear flight phases and different flight phases, and a roll slaving loop may be closed when the rate of turn is less in absolute value than a specified threshold.
When a loop is provided for compensating for the drifts, it is also possible to provide a slaving loop for correcting the gyrometric measurements along the vertical direction and this slaving loop comprises an integrator which is preferably common to the loop compensating for the drifts.
In one embodiment of this latter arrangement, the integrator comprises means making it possible to limit the mean slope of its output signal and means such that, at each instant, the output signal exhibits a discrepancy smaller than a threshold with respect to a linear-variation signal having a specified slope.